Dyslexia, or specific reading disability, is the most common childhood learning disorder. It is estimated that about 3-10% of people have specific difficulties in reading, despite adequate intelligence, education and social environment. Several different theories have been put forth to account for the diverse symptoms seen in dyslexic subjects. At present, it is thought that dyslexia is primarily a phonological deficit, emphasizing the linguistic basis of this condition (1-3). However, it is possible that dyslexia is not specific to language. Rather, it may result from a deficit in processing fast temporal data, be it visual or auditory. This temporal processing deficit would, consequently, manifest itself primarily as dyslexia (4).
Available evidence suggests that dyslexia is a neurological disorder with a genetic basis. Functional brain imaging studies have illustrated that dyslexia has universal neurobiological correlates (5). There is extensive evidence of genetic factors which contribute to dyslexia. There are significant differences, however, in the heritability of different components of dyslexia (6). Linkage and association studies have pinpointed several loci for dyslexia. In particular, two loci have been promising. DYX1 in chromosome 15q21 was the first locus to be associated with dyslexia (7), and the results have been replicated in three independent studies thereafter (8-10). The presence of a second dyslexia locus, DYX2, in chromosome 6p21 has also been established (11).
We have previously reported a translocation t(2; 15)(q11;q21) which segregates with dyslexia (12). In the present invention, we have cloned the breakpoint and narrowed down the breakpoint interval within a 3229 bp region with Southern hybridization. This region contains a 301 bp AT rich sequence. Considering that AT rich repeats are known to occur at many chromosomal rearrangement sites (13), the 301 bp AT rich sequence is likely to be the exact breakpoint site. Furthermore, we have unexpectedly discovered and characterized a novel gene residing in the breakpoint region, which gene we named DYXC1 and which is causally correlated with dyslexia.
A candidate gene for developmental dyslexia is disclosed in Taipale et al. (Proceedings of the National Academic of Sciences 100:11553-11558, 2003), which is incorporated herein by reference.